A 757 is not larger than a 747, which is what "Air Force One" is. I'm sort of surprised they didn't offer her a G-V, aka C-37A. That would have plenty of legs ( 6,300 miles) , but wouldn't be excessive (12 pax) , and they have more of them than C-20s anyway. C-20 has range of 4,250 miles, and also 12 pax. Andrews to San Francisco is 2,452 miles, so even with a headwind it can probably do the trick.
C-20B is good enough for the Speaker.
She wanted the same plane Condi takes to the Middle East; 42 passengers, a bedroom, entertainment system, communications top notch.
As somebody else said, she wants it so she and Murtha can tour the world without having to ask.
The plane she wanted would cost $300,000 RT to SFO.